The present invention relates to techniques for data transmission using acknowledgement mechanisms within the framework of automatic repeat request (ARQ) procedures.
It applies in particular, but not exclusively, to the hybrid (HARQ) ARQ procedures provided for in certain mobile radio communication systems.
In these techniques, the data are sent in blocks, and the receiver unit returns acknowledgement signals to the transmitting unit indicating which blocks have been improperly received and/or properly received. The acknowledgement may be positive (the properly received blocks are signalled), negative (the blocks not received, or received with insufficient reliability, are signalled) or binary (for each block, the receiving unit returns a positive or negative acknowledgement). After having tagged an improperly received block, the transmitting unit can effect a repetition of this block so as to palliate its initial improper reception, or send redundancy information so as to increase the probability of detection of the data.
Here the term “block” should be understood to mean the data unit forming the subject of an acknowledgement. It should be noted that such a data block can be sent in the form of several separate packets, on different physical resources or at different instants on the same physical resource.
Usually, the blocks sent are numbered, the number being present in a block header. The acknowledgement then identifies by its number the block to which it pertains. The transmitting unit therefore unambiguously identifies the blocks which are to form the subject of repeats.
In certain cases, the sequence number is not available at the level of the module of the receiving unit which processes the acknowledgement mechanism. This may occur when the acknowledgement mechanism is implemented in low layers of the OSI model, in particular in the physical layer.
In such a case, the identification of the blocks acknowledged positively or negatively can result from a certain synchronism between the receiving and transmitting units. Given that the acknowledgement signal relating to a block will be received in a time span determined after the transmission of this block, the transmitting unit can effect the association allowing it to determine the repeats to be performed.
It may happen that a transmission error adversely affects the acknowledgement signal returned by the receiving unit with respect to a block. For example, if a negative acknowledgement returned by the receiving unit is interpreted as a positive acknowledgement by the transmitting unit, the latter transmits a new block when the receiving unit waits for a repeat of the block previously improperly received.
This type of error poses a difficulty in the HARQ procedures in which, when the receiving unit receives a block improperly, it combines this reception (insufficient by itself) with the reception of the repeated or redundant block. Reception diversity is thus obtained, increasing the probability of proper reception of the data. However, an ambiguity regarding the identity of a block sent (sending of a new block or repeat) then leads to false recombinations. These errors are liable to persist since the receiving unit continues to request the resending of the ambiguous block.
It should be noted that various types of repetition and of combination of the repeated blocks may be used (see for example, WO 00/62467):    1/ the improperly received block may be repeated in full. Even if the repeat is also received improperly, it may be that a combination of the two successive observations of the block will allow the receiving unit to detect the data sent;    2/ this combination may be performed on the “hard” values of the symbols received (“hard combining”), or on their “soft values”, that is to say weighted by likelihood estimates (“soft combining”);    3/ the repeated block may be sent by applying a channel coding scheme different from the first send, with or without modification of the coding rate, thereby affording diversity of coding which may improve the probability of detection under hard or soft combining;    4/ the repeat may pertain only to a part of the block or may simply transport “incremental redundancy”. Such a redundancy block does not of itself allow the extraction of the data of the block initially sent, but it increases the probability of detection by a processing appropriate to the receiver.
To alleviate the difficulties related to possible improper reception of the acknowledgement signal, there may be provision for the transmitting unit to transmit with each block an indication regarding the type of this block, namely newly sent block or redundancy block subsequent to a negative acknowledgement.
When an acknowledgement signal is improperly received or improperly interpreted by the transmitting unit, this indication allows the receiving unit to be aware that a signalling error is involved. Nevertheless, an ambiguity regarding the nature of this error remains since the receiving unit cannot know whether the error has not occurred in the course of the sending of the block type indication. Given that the block accompanying the indication in accordance with which the receiving unit identifies an error is not the same depending on the direction of transmission in which this error is involved, the combining schemes, hard or soft, are also flawed.
An object of the present invention is to propose an acknowledgement and redundancy mechanism which limits the ambiguities regarding the identification of the blocks sent.